Extraction process for removal of impurities from mother liquor in the synthesis of carboxylic acid

ABSTRACT

A method for removing impurities from a mother liquor comprising a carboxylic acid, a metal catalyst, impurities by (a) evaporating the mother liquor comprising a carboxylic acid, a metal catalyst, impurities and a solvent in a first evaporator zone to produce a vapor stream and a concentrated mother liquor stream; (b) evaporating the concentrated mother liquor stream in a second evaporator zone to form a solvent rich stream and a super concentrated mother liquor stream; (c) mixing in a mixing zone a water-solvent solution and optionally an extraction solvent with the super concentrated mother liquor stream to form an aqueous mixture; (d) optionally separating organic impurities from the aqueous mixture in a solid-liquid separation zone to form a purified aqueous mixture; and (e) extracting the aqueous mixture or purified aqueous mixture with an extraction solvent in an extraction zone to form an extract stream and the raffinate stream.

FIELD OF INVENTION

This invention relates to the recovery of a metal catalyst from a motherliquor produced in the synthesis of carboxylic acid, typicallyterephthalic acid. More particularly, the process involves the additionof water to a super concentrated mother liquor stream to recover themetal catalyst and then subjecting an aqueous mixture or purifiedaqueous mixture so formed to a single stage extraction to remove organicimpurities to produce an extract stream and a raffinate streamcomprising the metal catalyst.

BACKGROUND OF THE INVENTION

Terephthalic acid is commercially produced by oxidation of paraxylene inthe presence of a catalyst, such as, for example, Co, Mn, Br and asolvent. Terephthalic acid used in the production of polyester fibers,films, and resins must be further treated to remove impurities presentdue to the oxidation of paraxylene.

Terephthalic acid (TPA) is an intermediate in the production ofpolyesters for plastics and fiber applications. Commercial processes forthe manufacture of TPA are based on the heavy-metal catalyzed oxidationof p-xylene, generally with a bromide promoter in acetic acid solvent.Due to the limited solubility of TPA in acetic acid under practicaloxidation conditions, a slurry of TPA crystals is formed in theoxidation reactor. Typically, the TPA crystals are withdrawn from thereactor and separated from the reaction mother liquor using conventionalsolid-liquid separation techniques. The mother liquor, which containsmost of the catalyst and promoter used in the process, is recycled tothe oxidation reactor. Aside from the catalyst and promoter, the motherliquor also contains dissolved TPA and many by-products and impurities.These by-products and impurities arise partially from minor impuritiespresent in the p-xylene feed stream. Other impurities arise due to theincomplete oxidation of p-xylene resulting in partially oxidizedproducts. Still other by-products result from competing side reactionsin the oxidation of p-xylene to terephthalic acid. The solid TPAcrystals obtained by solid-liquid separation are generally washed withfresh solvent to displace the major portion of the mother liquor andthen dried to remove most of the acetic acid solvent. The dried, crudeTPA crystals are contaminated with impurities that were present in themother liquor since these impurities are co-precipitated with the TPAcrystals. Impurities are also present due to occlusion in the TPAcrystal structure and due to incomplete removal of the mother liquor bythe fresh solvent wash.

Many of the impurities in the mother liquor that are recycled arerelatively inert to further oxidation. Such impurities include, forexample, isophthalic acid, phthalic acid and trimellitic acid.Impurities, which undergo further oxidation are also present, such as,for example, 4-carboxybenzaldehyde, p-toluic acid and p-tolualdehyde.The concentration of oxidation inert impurities tends to accumulate inthe mother liquor. The concentration of these inert impurities willincrease in the mother liquor until an equilibrium is reached wherebythe amount of each impurity contained in the dry TPA product balancesits rate of formation or addition to the oxidation process. The normallevel of impurities in crude TPA makes it unsuitable for direct use inmost polymer applications.

Traditionally, crude TPA has been purified either by conversion to thecorresponding dimethyl ester or by dissolution in water with subsequenthydrogenation over standard hydrogenation catalysts. More recently,secondary oxidative treatments have been used to produce polymer-gradeTPA. Irrespective of the method used to purify TPA to render it suitablefor use in polyester manufacture, it is desirable to minimize theconcentrations of impurities in the mother liquor and thereby facilitatesubsequent purification of TPA. In many cases, it is not possible toproduce a purified, polymer-grade TPA unless some means for removingimpurities from the mother liquor is utilized.

One technique for impurity removal from a recycle stream commonly usedin the chemical processing industry is to draw out or “purge” someportion of the recycle stream. Typically, the purge stream is simplydisposed of or, if economically justified, subjected to varioustreatments to remove undesired impurities while recovering valuablecomponents. One example is U.S. Pat. No. 4,939,297 herein incorporatedby reference. The amount of purge required for control of impurities isprocess-dependent; however, a purge amount equal to 10-40% of the totalmother liquor is usually sufficient for TPA manufacture. In theproduction of TPA, the level of mother liquor purge necessary tomaintain acceptable impurity concentrations, coupled with the higheconomic value of the metal catalyst and solvent components of themother liquor, make simple disposal of the purge stream economicallyunattractive. Thus, there is a need for a process that recoversessentially all of the expensive metal catalysts and acetic acidcontained in the mother liquor while removing a major portion of theimpurities present in the purge stream. The metal catalyst should berecovered in an active form suitable for reuse by recycling to thep-xylene oxidation step.

This invention is a marked improvement over a typical purge process.Some of the advantages are:

-   -   1) enhanced operability and reliability due to reduction in        plugging potential; and    -   2) reduction in overall energy usage.

The invention enhances the impurity removal efficacy of the process, andthe operability of the process compared to the existing processes.

SUMMARY OF THE INVENTION

This invention relates to removal of impurities and the recovery of ametal catalyst from mother liquor produced in the synthesis ofcarboxylic acid, typically terephthalic acid. More particularly, theprocess involves the addition of water to a concentrated mother liquorto recover the metal catalyst and then subjecting an aqueous mixture soformed to a single stage extraction to remove organic impurities toproduce an extract stream and a raffinate stream.

It is an object of this invention to provide a process to produce asuper concentrated mother liquor stream.

It is yet another object of this invention to provide a process torecover a metal catalyst from a mother liquor stream.

It is yet another object of this invention to provide a process forremoval of impurities and the recovery of a metal catalyst from motherliquor produced in the synthesis of carboxylic acid.

In a first embodiment of this invention, a process to recover a metalcatalyst from a mother liquor is provided. The process comprises thefollowing steps:

-   -   (a) evaporating a mother liquor comprising a carboxylic acid,        the metal catalyst, impurities, water and a solvent in a first        evaporator zone to produce a vapor stream and a concentrated        mother liquor stream;    -   (b) evaporating the concentrated mother liquor stream in a        second evaporator zone to form a solvent rich stream and a super        concentrated mother liquor stream;    -   (c) mixing in a mixing zone a water-solvent solution with the        super concentrated mother liquor stream to form an aqueous        mixture;    -   (d) optionally separating organic impurities from the aqueous        mixture in a solid-liquid separation zone to form a purified        aqueous mixture;    -   (e) adding an extraction solvent to the aqueous mixture or the        purified aqueous mixture in an extraction zone to form an        extract stream and a raffinate stream; and    -   (f) optionally separating the extract stream and the solvent        rich stream in a separation zone to form a high boiling point        organic impurities stream and a recovered extraction solvent        stream.

In another embodiment of this invention, a process to produce a superconcentrated mother liquor stream is provided. The process comprises thefollowing steps:

-   -   (a) evaporating a mother liquor comprising a carboxylic acid, a        metal catalyst, impurities, water and a solvent in a first        evaporator zone to produce a vapor stream and a concentrated        mother liquor stream; and    -   (b) evaporating the concentrated mother liquor stream in a        second evaporator zone to produce a solvent rich stream and a        super concentrated mother liquor stream wherein the evaporating        in step (a) and step b) combined removes about 95 wt % to about        99 wt % of the water and solvent from the mother liquor.

In another embodiment of this invention, a process to recover a metalcatalyst from a mother liquor is provided. The process comprises thefollowing steps:

-   -   (a) evaporating a mother liquor comprising a carboxylic acid,        the metal catalyst, impurities, water and a solvent in a first        evaporator zone to produce a vapor stream and a concentrated        mother liquor stream;    -   (b) evaporating the concentrated mother liquor stream in a        second evaporator zone to form a solvent rich stream and a super        concentrated mother liquor stream;    -   (c) mixing in a mixing zone a water-solvent solution and        optionally an extraction solvent with the super concentrated        mother liquor stream to form an aqueous mixture;    -   (d) adding an extraction solvent to the aqueous mixture in an        extraction zone to form an extract stream and the raffinate        stream; and    -   (e) optionally separating the extract stream and the solvent        rich stream in a separation zone to form a high boiling point        organic impurities stream and a recovered extraction solvent        stream.

In another embodiment of the invention, a process to recover a metalcatalyst from a mother liquor is provided. The process comprises thefollowing steps:

-   -   (a) evaporating the mother liquor in a first evaporator zone to        produce a vapor stream and a concentrated mother liquor stream        wherein about 50 wt % to about 80 wt % of the solvent and water        is removed from the mother liquor;    -   (b) evaporating the concentrated mother liquor stream in a        second evaporator zone to form a solvent-rich stream and a super        concentrated mother liquor stream wherein about 95 wt % to about        99 wt % of solvent and water are removed from the mother liquor        in the combined step (a) and step (b);    -   (c) mixing a water-solvent solution with the super concentrated        mother liquor stream to form an aqueous mixture; wherein the        water-solvent solution is added to quench the aqueous mixture to        a temperature range of about 60° C. to about 95° C.;    -   (d) optionally separating organic impurities from the aqueous        mixture in a solid-liquid separation zone to form a purified        aqueous mixture;    -   (e) extracting the aqueous mixture or the purified aqueous        mixture with an extraction solvent in an extraction zone to form        an extract stream and a raffinate stream wherein the extraction        zone comprises a single stage extractor; and    -   (f) separating the extract stream and the solvent rich stream in        a separation zone to form a high boiling point organic        impurities stream and a recovered extraction solvent stream.

In another embodiment of the invention, a process to recover a metalcatalyst from a mother liquor is provided. The process comprises thefollowing steps:

-   -   (a) evaporating the mother liquor in a first evaporator zone to        produce a vapor stream and a concentrated mother liquor stream        wherein about 50 wt % to about 80 wt % of the solvent and water        is removed from the mother liquor;    -   (b) evaporating the concentrated mother liquor stream in a        second evaporator zone to form a solvent-rich stream and a super        concentrated mother liquor stream wherein about 95 wt % to about        99 wt % of solvent and water are removed from the mother liquor        in the combined step (a) and step (b);    -   (c) mixing in a mixing zone a water-solvent solution and        optionally an extraction solvent with the super concentrated        mother liquor stream to form an aqueous mixture; wherein the        water-solvent solution is added to quench the aqueous mixture to        a temperature range of about 60° C. to about 95° C.;    -   (d) extracting the aqueous mixture or the purified aqueous        mixture with an extraction solvent in an extraction zone to form        an extract stream and a raffinate stream wherein the extraction        zone comprises a single stage extractor; and    -   (e) separating the solvent organic impurities stream and the        solvent rich stream in a separation zone to form a high boiling        point organic impurities stream and a recovered extraction        solvent stream.

In another embodiment of this invention, a composition is provided. Thecomposition comprises acetic acid, water, isophthalic acid, benzoicacid, 4-carboxybenzaldehyde, terephthalic acid, and cobalt; wherein thesum aggregrate of the isophthalic acid, benzoic acid,4-carboxybenzaldehyde, and terephthalic acid comprise between about 5 wt% to 80% of the concentrated mother liquor. These objects, and otherobjects, will become more apparent to others with ordinary skill in theart after reading this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates different embodiments of the invention where aprocess to recover a metal catalyst from a mother liquor and a processto produce a super concentrated mother liquor stream are provided.

DESCRIPTION OF THE INVENTION

In one embodiment of this invention, a process to recover a metalcatalyst from a mother liquor 301 is provided as shown in FIG. 1. Theprocess comprises the following steps.

Step (a) comprises evaporating a mother liquor 301 comprising acarboxylic acid, the metal catalyst, impurities, water and a solvent ina first evaporator zone 321 to produce a vapor stream 304 and aconcentrated mother liquor stream 305.

The mother liquor 301 is withdrawn from a carboxylic acid oxidativesynthesis process. The mother liquor 301 serves as the feed stream tothe present process. The mother liquor comprises carboxylic acid, water,a solvent, the metal catalyst and impurities. The impurities compriseorganic bromides and corrosion metals. The organic bromides are used aspromoters in the oxidation reaction. Examples of corrosion metals areiron and chromium compounds, which inhibit, reduce or entirely destroythe activity of the metal catalyst.

Suitable carboxylic acids are selected from the group consisting ofterephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, andmixtures thereof.

Suitable solvents include aliphatic mono-carboxylic acids, preferablycontaining 2 to 6 carbon atoms, or benzoic acid and mixtures thereof andmixtures with water. Preferably, the solvent is acetic acid mixed withwater, in a ratio of about 5:1 to about 25:1, preferably between about10:1 and about 15:1. Throughout the specification, acetic acid will bereferred to as the solvent. However, it should be appreciated that othersuitable solvents, such as those disclosed here, may also be utilized.

In the first step of the present process, the mother liquor isconcentrated by conventional means in a first evaporator zone 321comprising an evaporator to produce a vapor stream 304 and aconcentrated mother liquor stream 305. The evaporator is operated atatmospheric or slightly superatmospheric conditions, generally fromabout 1 atmosphere to about 10 atmospheres. The vapor stream 304comprises a majority of the water and solvent, and the concentratedmother liquor stream 305 comprises the remainder of the water andsolvent not removed from the mother liquor. The evaporation removesabout 50 wt % to about 80 wt % of the solvent and water, typicallyacetic acid and water, which are present in the mother liquor.

Step (b) comprises evaporating the concentrated mother liquor stream 305in a second evaporator zone 350 to produce a solvent rich stream 344 anda super concentrated mother liquor stream 345.

The concentrated mother liquor stream 305 is then introduced in thesecond evaporator zone 350, which comprises at least one evaporator. Theevaporator is operated at super atmospheric or pressurized conditions,generally from about 1 atmosphere to about 10 atmospheres. Theevaporation is conducted at a temperature from about 150° C. to about220° C.; another range is from about 180° C. to about 200° C. Thecombination of evaporators 321 and 350 are operated so as to concentratethe mother liquor 301 as represented by stream 301 to a conditionwherein 95-99 wt % of the solvent, typically acetic acid and water, isremoved from the mother liquor 301.

In the present process, the condition of the super concentrated motherliquor stream 345 is as a high temperature molten dispersion with onlyenough remaining solvent to provide pumpability. In one embodiment, atypical composition of the super concentrated mother liquor 345 is shownin Table 1. Generally, the mass composition of the sum total of allcompounds shown in Table 1, excluding water and acetic acid, in thesuper concentrated mother liquor 345 can vary between about 5 wt % toabout 80 wt % based on the total weight of the super concentrated motherliquor 345. Another range for the sum total of all compounds shown inTable 1, excluding acetic acid and water, in the super concentratedmother liquor 345 can be all combinations of upper and lower rangeswhere the lower ranges are 5 wt %, 10 wt %, 15 wt %, 20 wt %, 25 wt %,30 wt %, 35 wt %, 40 wt % and the upper ranges are 80 wt %, 75 wt %, 70wt %, 65 wt %, 60 wt %, 55 wt %, 50 wt %, 45 wt % based on the totalweight of the super concentrated mother liquor 345. Further, rangesstated in this disclosure and the claims that follow should beunderstood to disclose the entire range specifically and not just theend point(s). For example, disclosure of the range 0 to 10 should betaken to specifically disclose 2, 2.5, 3.17 and all other numbersubsumed and not just 0 and 10.

Step (c) comprises mixing in a mixing zone 348 a water-solvent solution306 with the super concentrated mother liquor stream 345 to form anaqueous mixture 307.

The super concentrated mother liquor steam 345 is then subjected toextraction of the metal catalyst in the mixing zone 348 by introductionof a water-solvent solution 306 which can contain water or awater-acetic acid or a water-solvent solution to form an aqueous mixturein stream 307 wherein at least 80% of the metal catalyst is recovered inthe aqueous phase of the aqueous mixture 307. Typically, at least 90% ofthe metal catalyst is recovered in the aqueous phase of the aqueousmixture 307. The water-solvent solution comprises water and optionallyan additional solvent. The solvent can be any substance capable ofdissolving the metal catalyst to form a uniformly dispersed solution atthe molecular or ionic size level. Typically, the solvent comprisesacetic acid, but solvents that have been previously mentioned in step(a) can also be utilized.

The mixing zone 348 comprises a vessel and/or device or a plurality ofvessels or devices wherein there is sufficient residence time for themetal catalyst and/or halogen compounds (e.g. bromine) to dissolve intosolution. Examples of such vessels are devices include, but are notlimited to, a tank and a stirred or agitated tank. In this step, it isnot necessary to completely dissolve the mixture. One method is toutilize only the necessary amount of water to obtain the level of themetal catalyst recovery desired. However, the addition of water solventsolution 306 also serves to quench the mixture to a temperatures in therange of about 60° C. to about 95° C., another range is about 80° C. toabout 90° C. The quenching is done for about 0.5 to about 4 hours,another range is about 1 to about 2 hours. By this treatment organicbromides are reacted to yield inorganic bromides that are for example,preferentially retained in the aqueous fraction exiting an extractor.The quantity of bromine-containing compounds purged from the systemalong with the unwanted impurities is thereby minimized. The heattreatment conserves bromides and simplifies disposal of the organicimpurities.

The addition of water in the mixing zone 348 not only recovers the metalcatalyst in the super concentrated mother liquor, but also aids inpumping the aqueous mixture 307. It is desirable to keep the aqueousmixture 307 circulating with an external circulation loop.

In one embodiment, a typical composition of the aqueous mixture is shownin Table 1. Generally, the mass composition of the aqueous mixture 307in this embodiment generally can vary wherein the mass ratio of water toacetic acid is in the range of about 1:1 to 99:1 and wherein the sumaggregate of isophthalic acid, benzoic acid, 4-carboxybenzaldehyde, andterephthalic comprises between about 1000 ppm to about 65 wt % of thetotal weight of the aqueous mixture 307. Another range can be allcombinations of upper and lower ranges wherein the sum aggregate ofisophthalic acid, benzoic acid, 4-carboxybenzaldehyde, and terephthalichave a lower range of 5 wt %, 10 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt%, 35 wt %, 40 wt % and a upper range of 65 wt %, 60 wt %, 55 wt %, 50wt %, 45 wt % based on the total weight of the aqueous mixture 307.

When separating in the solid liquid separation zone 351 is performed, asmall amount of extraction solvent in conduit 311, generally about 1 toabout 10% by weight, preferably about 5% by weight, may be added to themixing zone 348 to enhance slurry handling by reducing adherence ofsolids to the side of, for example, a slurry feed tank. This isrepresented by the dashed arrow from stream 311 in FIG. 1.

Step (d) comprises optionally separating organic impurities 312 from theaqueous mixture 307 in a solid-liquid separation zone 351 to form apurified aqueous mixture 308.

The aqueous mixture stream 307 can be optionally fed to a solid-liquidseparation zone comprising a solid-liquid apparatus, 351, whereinorganic impurities 312 may be removed from the aqueous mixture 307 toform a purified aqueous mixture 308 and organic impurities 312. Thereare no limitations on the type of solid-liquid separation apparatus aslong as it is sufficient to remove organic impurities 312 from theaqueous mixture 307. Examples of such apparatuses include, but are notlimited to, filters, centrifuges, cyclones, hydroclones, etc.

The organic impurities can comprise numerous compounds typicallyassociated with TPA production. Examples of typical organic impuritiesinclude, but are not limited to, isophthalic acid, trimellitic acid,benzoic acid, phthalic acid, fluorenones compounds, p-toluic acid, and4-carboxybenzaldehyde. In one embodiment, a typical composition of thepurified aqueous mixture 308 is shown in Table 1. The mass compositionof the purified aqueous mixture 308 in this embodiment comprises aceticacid, water, isophthalic acid, benzoic acid, 4-carboxybenzaldehyde,terephthalic acid, and cobalt; wherein the sum aggregate of theisophthalic acid, benzoic acid, 4-carboxybenzaldehyde, and terephthalicacid comprise between about 1 wt % to 70% based on the total weight ofthe purified aqueous mixture 308; wherein the sum aggregate ofisophthalic acid and terephthalic acid comprise no more than 10 wt % ofthe purified aqueous mixture 308. Another range can be all combinationsof upper and lower ranges wherein the sum aggregate of isophthalic acid,benzoic acid, 4-carboxybenzaldehyde, and terephthalic have a lower rangeof 5 wt %, 10 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt %, 35 wt %, 40 wt %based on the total weight of the purified aqueous mixture 308 and aupper range of 65 wt %, 60 wt %, 55 wt %, 50 wt %, 45 wt % based on thetotal weight of the purified aqueous mixture 308; and wherein the sumaggregate of isophthalic acid and terephthalic acid comprise no morethan 10 wt % based on the total weight of the purified aqueous mixture308.

As previously stated when the solid-liquid separation zone 351 isutilized, a small amount of extraction solvent in conduit 311, generallyabout 1 to about 10% by weight, preferably about 5% by weight may beadded to the mixing zone 348 to enhance slurry handling by reducingadherence of solids to the side of, for example, a slurry feed tank.This is represented by the dashed arrow from stream 311 in FIG. 1.

Step (e) comprises adding an extraction solvent 311 to the aqueousmixture 307 or the purified aqueous mixture 308 in an extraction zone323 to form an extract stream 309 and the raffinate stream 310.

The aqueous mixture 307 or the purified aqueous mixture 308 is fed to anextraction zone 323 wherein the aqueous mixture 307 or the purifiedaqueous mixture 308 and the extraction solvent 311 are contacted in theextraction zone 323. The aqueous mixture 307 or the purified aqueousmixture 308 and the extraction solvent 311 are mixed to form an extractstream 309 comprising solvent, water organic impurities, and organicsolvent which forms a lighter phase, and the raffinate stream 310comprising a metal catalyst, corrosion metals, and water. The extractstream 309 is withdrawn as an overhead stream, and the raffinate stream310 is withdrawn from the bottom of extractor in the extraction zone323. In this invention, one embodiment of the extraction zone 323 is asingle stage extractor. In an embodiment of the invention, theextraction zone comprises a counter current extractor.

The extraction solvent 311 used in the extractor should be substantiallywater-insoluble to minimize the amount of organic solvent dissolved inthe aqueous fraction. Additionally, the extraction solvent 311 ispreferably an azeotropic agent which serves to assist solvent recoveryfrom the organic extract. Solvents, which have proven to be particularlyuseful are C1 to C6 alkyl acetates, particularly n-propyl acetate(n-PA), isopropyl acetate, isobutyl acetate, sec-butyl acetate, ethylacetate and n-butyl acetate, although other water-insoluble organicsolvents having an appropriate density and a sufficiently low boilingpoint may also be used, such as p-xylene. N-propyl acetate and isopropylacetate are particularly preferred due to their relatively low watersolubility, excellent azeotropic behavior, and their ability to removethe remaining acetic acid as well as high-boiling organic impuritiesfrom the aqueous mixture.

The extraction can be effected using extraction solvent ratios fromabout 1 to about 4 parts by weight extraction solvent per part ofextractor feed depending on the extractor feed composition. Spacevelocities of the combined feeds to the extractor generally range fromabout 1 to about 3 hr⁻¹. Although the extraction can be conducted atambient temperature and pressure, heating the extraction solvent 311 andextractor to about 30°-70° C. Another range of about 40° C. to about 60°C. can be used. Although the extract stream 309 comprises small amountsof the metal catalyst and corrosion metals, essentially all of the metalcatalyst and the majority of the remaining corrosion metals arecontained in the heavier phase, the raffinate stream 310.

Step (f) comprises optionally separating the extract stream 309 and thesolvent rich stream 344 in a separation zone 324 to form a high boilingpoint organic impurities stream 315 and a recovered extraction solventstream 317.

The extract stream 309 comprises organic solvent and organic impurities.The extract stream 309 can further comprises acetic acid and water,often in minor amounts. The extract stream 309 may be distilled in aseparation zone comprising conventional distillation equipment. Thedistillation equipment is operated at process conditions sufficient torecover a majority of the extraction solvent, typically n-propylacetate, from the extract stream 309 into the recovered extractionsolvent stream 317. Convention distillation equipment includes, forexample, a distillation column. One key feature to this invention is there-introduction of the solvent rich stream 344 into the separation zone324.

Most of the organic impurities are extracted by the organic solvent inthe extraction zone 323. This occurs because the organic impurities showa high degree of solubility for the organic solvent and to a lesserextent for acetic acid. By distilling the lighter phase from theextractor, the organic solvent is evaporated allowing the organicimpurities to concentrate in the column underflow. This results in ahigh probability for plugging and precipitation of solids. By utilizingthe solvent rich stream 344, the organic impurities in the columnunderflow can be effectively diluted and thereby solubilized by aceticacid in the column underflow. In an embodiment of the invention, thesolvent rich stream 344 comprises a solvent selected from the groupconsisting of n-propyl acetate, isopropyl acetate, isobutyl acetate,sec-butyl acetate, ethyl acetate and n-butyl acetate.

The use of the solvent rich stream 344, from the previous evaporationserves two functions. First, the loss of the organic solvent isminimized since the solvent rich stream 344 effectively displaces theorganic solvent in the column underflow. Second, the use of acetic-acidrich vapor provides significant enthalpy needed for driving thedistillation/separation process.

The separation zone 324 will need to process significantly lesshydraulic load than a typical purge process due to the greaterconcentration of mother liquor. Recovered extraction solvent and aceticacid may be recycled to the extractor and oxidative reactor,respectively. The high-boiling organic impurities are removed as sludgefrom the base of the distillation column for disposal.

Although the composition of the various streams in the process variesdepending on the process conditions, a typical composition of thestreams are shown in Table 1. In Table 1, the components are shown inthe left hand column and the amount of these components in each streamin the FIG. 1 are shown in the number column corresponding to the numberof the stream in FIG. 1. The amounts of the components shown in Table 1can be any measurement of weight as long as it is consistent for allcomponents and all streams. For example, the mother liquor 301 hasacetic acid in the amount of 915 pounds, 915 grams, etc.

TABLE 1 Material Balance Process Material Balance Stream in FIG. 1 301304 305 344 345 306 307 308 309 310 311 312 Acetic Acid 915.0 534.1380.9 335.2 45.8 — 45.8 45.3 44.1 1.2 — 0.4 Water 55.0 39.3 15.7 14.71.0 80.0 81.0 80.2 35.6 44.5 — 0.7 n-Propyl Acetate — — — — — — — —399.0 1.0 400.0 — Terephthalic Acid 0.71 — 0.71 — 0.71 — 0.71 0.70 0.70— — — Isophthalic Acid 5.83 — 5.83 — 5.83 — 5.83 5.78 5.71 0.07 — 0.05Phthalic Acid 3.81 — 3.81 0.12 3.69 — 3.69 3.66 3.36 0.29 — 0.03 BenzoicAcid 8.12 0.06 8.06 2.27 5.79 — 5.79 5.73 5.73 — — 0.054-Carboxybenzaldehyde 1.56 — 1.56 — 1.56 — 1.56 1.54 1.52 0.02 — 0.01Trimellitic Acid 1.17 — 1.17 — 1.17 — 1.17 1.16 1.01 0.14 — 0.01Paratoluic Acid 2.96 0.01 2.95 0.50 2.44 — 2.44 2.42 2.39 0.03 — 0.02Paratolualdehyde 0.51 0.05 0.46 0.26 0.20 — 0.20 0.20 0.20 — — — Others2.50 — 2.50 — 2.50 — 2.50 2.38 2.14 0.24 — 0.13 Organic Bromide 1.30 —1.30 — 1.30 — 0.90 0.86 — 0.85 — 0.05 Ionic Bromide 0.34 — 0.34 — 0.34 —0.74 0.70 — 0.70 — 0.04 Cobalt 1.44 — 1.44 — 1.44 — 1.44 1.37 0.01 1.35— 0.07 Manganese 0.10 — 0.10 — 0.10 — 0.10 0.10 — 0.09 — — CorrosionMetals 0.08 — 0.08 — 0.08 — 0.08 0.08 — 0.08 — — Total 1000 573 427 35374 80 154 152 502 51 400 2 *The amounts of the components shown in Table1 can be any measurement of weight as long as it is consistent for allcomponents and all streams

1. A process comprising: (a) evaporating a mother liquor comprising acarboxylic acid, a metal catalyst, impurities, water and a solvent in afirst evaporator zone to produce a vapor stream and a concentratedmother liquor stream; (b) evaporating said concentrated mother liquorstream in a second evaporator zone to form a solvent rich stream and asuper concentrated mother liquor stream; wherein said super concentratedmother liquor stream is a high temperature molten dispersion; whereinsaid evaporating in said second evaporator zone is conducted at atemperature from about 150° C. to about 220° C.; (c) mixing in a mixingzone a water-solvent solution and optionally an extraction solvent withsaid super concentrated mother liquor stream to form an aqueous mixture;(d) adding an extraction solvent to said aqueous mixture in anextraction zone to form an extract stream and a raffinate stream; and(e) separating said extract stream and said solvent rich stream in aseparation zone to form a high boiling point organic impurities streamand a recovered extraction solvent stream.
 2. The process according toclaim 1 wherein about 50 wt % to about 80 wt % of said solvent isremoved from said mother liquor in step (a).
 3. The process according toclaim 1 wherein about 95 wt % to about 99 wt % of said solvent and wateris removed from said mother liquor in step (a) and step (b) combined. 4.The process according to claim 1 wherein said extraction solvent isoptionally added in step (c) in an amount of about 1 to about 10 percentby weight.
 5. The process according to claim 1 wherein saidwater-solvent solution is added to quench said aqueous mixture to atemperature range of about 60° C. to about 95° C.
 6. A process accordingto claim 1 wherein said extraction zone comprises a counter currentextractor.
 7. A process according to claim 1 wherein said extractionzone comprises a single stage extractor.
 8. A process according to claim1 wherein said solvent rich stream comprises a solvent selected from thegroup consisting of n-propyl acetate, isopropyl acetate, isobutylacetate, sec-butyl acetate, ethyl acetate and n-butyl acetate.
 9. Aprocess according to claim 1 wherein said second evaporator zonecomprises an evaporator operated at a temperature of about 150° C. toabout 200° C.
 10. A process according to claim 1 where said firstevaporator zone is operated at a pressure of greater that 14.6 psia. 11.A process comprising: (a) evaporating a mother liquor comprising acarboxylic acid, a metal catalyst, impurities, water and a solvent in afirst evaporator zone to produce a vapor stream and a concentratedmother liquor stream; (b) evaporating said concentrated mother liquorstream in a second evaporator zone to form a solvent rich stream and asuper concentrated mother liquor stream; wherein said super concentratedmother liquor stream is a high temperature molten dispersion; whereinsaid evaporating in said second evaporator zone is conducted at atemperature from about 150° C. to about 220° C.; (c) mixing in a mixingzone a water-solvent solution with said super concentrated mother liquorstream to form an aqueous mixture; (d) separating organic impuritiesfrom said aqueous mixture in a solid-liquid separation zone to form apurified aqueous mixture; (e) adding an extraction solvent to saidaqueous mixture or said purified aqueous mixture in an extraction zoneto form an extract stream and a raffinate stream; and (f) separatingsaid extract stream and said solvent rich stream in a separation zone toform a high boiling point organic impurities stream and a recoveredextraction solvent stream.
 12. The process according to claim 11 whereinabout 50 wt % to about 80 wt % of said solvent is removed from saidmother liquor in step (a).
 13. The process according to claim 11 whereinsaid water-solvent solution is added to quench said aqueous mixture to atemperature range of about 60° C. to about 95° C.
 14. The processaccording to claim 11 wherein said water-solvent solution is added toquench said aqueous mixture to a temperature range of 80° C. to about90° C.
 15. The process according to claim 11 wherein said extractionzone comprises a counter current extractor.
 16. The process according toclaim 11 wherein said extraction zone comprises a single stageextractor.
 17. The process according to claim 11 wherein said solventrich stream comprises a solvent selected from the group consisting ofn-propyl acetate, isopropyl acetate, isobutyl acetate, sec-butylacetate, ethyl acetate and n-butyl acetate.